FWIW, mksh has three different “echo”; if invoked as mksh, it uses
a BSD echo by default which does interpret backslashes, but if one
uses set -o posix (or invokes it as sh or -sh and it is compiled
with -DMKSH_BINSHPOSIX (CVS HEAD)) it has an echo that only honours
-n as first argument and doesn’t do anything else (Debian Policy §10.4).